It will be interesting to see how they solve the mystery of why (to my knowledge) all of the sites having clays or other water-modified rocks and minerals are found in the ancient cratered highlands and not in the smooth northern plains

--------------------

I'm one of the most durable and fervent advocates of space exploration, but my take is that we could do it robotically at far less cost and far greater quantity and quality of results.

That explains very well why the underlying strata is heavily cratered, but covered by the smooth northern basin. Given the Comanche find and the ph at the Phoenix site could we anticipate that the ocean was a neutral or high ph and that there are carbonates down there? Or is that too much of a leap of faith.

There are many processes and environments involved in the formation of carbonates. Until we know more about the type of carbonate involved and the depositional environment, we don't know many of the fine details. It's just one data point, one piece of the jigsaw puzzle.

My take on Comanche is that it is a hydrothermal feature, whci is a whole 'nuther critter from aquatic deposition.

There are likely at least two different "oceans" in Mars history. The DiAchille paper, cited in Bobby's post, analyzed valley networks and deltas from the Noachian period (>3.5 billion years ago.) when climatic conditions were very different than today. Some temporary bodies of water (or mud) probably formed much later in the northern hemisphere in association with the hugh outflow channels carved during the Hesperian (maybe 500 to 1,000 million years after the earlier ocean--absolute dates are uncertain). The chemical environments (pH etc.) were likely very different during each.

What's puzzling to me is that an ancient, cratered Noachian surface is preserved at realtively shallow depths in the northern hemisphere, suggesting that the sediment layers from both oceanic events are fairly thin. Mars is such a fascinating enigma.

The ocean froze and we see the stuff deposited on top, not the seabed sediments?

If there were seabed sediments in the northern plains, they're certainly not what's at the surface right now. Young craters on the northern plains have very bouldery ejecta, which implies that the material they impacted into is hard like a basalt rather than soft marine sediments.

There are likely at least two different "oceans" in Mars history. The DiAchille paper, cited in Bobby's post, analyzed valley networks and deltas from the Noachian period (>3.5 billion years ago.) when climatic conditions were very different than today. Some temporary bodies of water (or mud) probably formed much later in the northern hemisphere in association with the hugh outflow channels carved during the Hesperian (maybe 500 to 1,000 million years after the earlier ocean--absolute dates are uncertain). The chemical environments (pH etc.) were likely very different during each.

What's puzzling to me is that an ancient, cratered Noachian surface is preserved at realtively shallow depths in the northern hemisphere, suggesting that the sediment layers from both oceanic events are fairly thin. Mars is such a fascinating enigma.

Tom

My own current thinking on this subject is that the ocean grew after accretion, reaching it's maximum extent in the late Noachian, was initially largely unfrozen, then was frozen over and covered with debris as it monotonically declined over time, with transgressive pulses producing the "shorelines" I identified in Viking images. The ocean is still there, but likely frozen solid (and less extensive).

Blocks at the surface in the putative ocean basin interior can be explained in a number of ways. "Sorting" and concentrating of blocks at the surface through periglacial processes; Marine sediment layer overlying basement rocks (and craters) is thin or absent; fines have been selectively winnowed out by aeolian (or aqueous) processes.

If you dried up the Earth's oceans, much of the exposed ocean floor would be basalt.

Thanks for clarifying, Tim.I was thinking oceans (plural) in terms of the differing chemistry, areal extent, and climatic conditions under which standing bodies of water may have formed. Given how ancient the dichotomy appears to be, it seems reasonable that the northern lowlands have been the major sink for water throughout Martian history, and as such would have been the location of the true martian "ocean" (ultimate base level) since very early in the Noachian.

(1) Mars *had to* have a lot of water early on and thus, had to have fairly extensive ocean.(2) But we don't see much of sedimentary rocks.

One crazy-ish idea from me: what if Mars indeed had a lot of water, but it always was mostly solid?

Even here on Earth, with beefy atmosphere (-> greenhouse effect) and higher insolation, we had pretty bad glaciations. At Mars distance, it should have been colder. What if Mars "ocean" was mostly frozen solid, like continuous ice age with only short periods of catastrophic melting and floods, when polar tilt, greenhouse effect and volcanic heat happen to work together?

After billions of years of slow water loss, especially from surface closer to equator, this will give us today outwardly "dirty", but inside pretty "icy" Mars (lots of ancient river/flood valleys, glaciers, lots of permafrost at latitudes +/-40 and up to poles), yet not much of sediments. Sedimentation doesn't happen in solid water, right?

From one, given that solar output has increased by several tens of percent since the birth of the solar system, an early Mars - one with noticeably less insolation - could have been an appreciably colder planet than today's, thus supporting this idea.

From the other, I suppose an early Mars' thicker atmosphere and more tectonic activity works in favour of warming the place up.

Quite which chair wins is not something I feel at all qualified to comment on.

Actually are there not 3 variables (armchairs) to consider? 1. Output from the sun (insolation at the upper atmosphere) increasing with time, 2. Atmospheric make-up and heat from impact/volcanic activity (decreasing with time), and3. Distance from the sun (given the apparent instability of the early solar system as evidenced by the LHB, possibly increasing early on).

Ok - just a blip at the edge of the probability distribution but isn't a warmer wetter Mars a possibility worth consideration?

One crazy-ish idea from me: what if Mars indeed had a lot of water, but it always was mostly solid?

Even here on Earth, with beefy atmosphere (-> greenhouse effect) and higher insolation, we had pretty bad glaciations. At Mars distance, it should have been colder. What if Mars "ocean" was mostly frozen solid, like continuous ice age with only short periods of catastrophic melting and floods, when polar tilt, greenhouse effect and volcanic heat happen to work together?

After billions of years of slow water loss, especially from surface closer to equator, this will give us today outwardly "dirty", but inside pretty "icy" Mars (lots of ancient river/flood valleys, glaciers, lots of permafrost at latitudes +/-40 and up to poles), yet not much of sediments. Sedimentation doesn't happen in solid water, right?

Not crazy at all. Your scenario resembles the one that Jeff Kargel discusses in Mars: A Warmer Wetter Planet.

Surface water on Earth is mostly liquid with with ocassional climactic excursions producing widespread surface ice (glaciations). Mars, being colder, is the opposite: Water is frozen most of the time and only ocassionally do conditions exist to produce liquid water. Such conditions became much less frequent and of lower intensity over time.

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